1. Field of the Invention
The present invention relates to hydraulic systems for operating machinery, and in particular to electronic circuits that operating valves to control the flow of fluid in such hydraulic systems.
2. Description of the Related Art
A wide variety of machines have components that are moved by an hydraulic actuator, such as a cylinder and piston arrangement, which is controlled by a valve assembly. Traditionally a manually operated spool type hydraulic valve was used to control the fluid flow to and from the actuator. There is a present trend toward electrical controls and the use of solenoid operated valves. With this type of control, pressurized fluid from a pump is applied to one chamber of the hydraulic cylinder by opening a first solenoid operated, proportional poppet valve and at the same time a second solenoid operated, proportional poppet valve is opened to allow the fluid in the other cylinder chamber to flow back to the system tank.
When those valves close, i.e. when motion of the piston in the hydraulic cylinder is not desired, pressure often becomes trapped in the cylinder chambers thereby affecting the workport pressure at the valve assembly. Trapped pressure of a significant magnitude can produce undesired motion when the valves reopen to activate the hydraulic actuator again. For example, load “droop” when the trapped pressures are released when both valves are modulated without taking the initial workport pressure into account. Depending upon the trapped workport pressure states, supply and return pressures, the metering mode and the direction of the commanded motion, the condition can result in the piston initially moving slightly in the wrong direction when small magnitudes of fluid flow are being sent to the hydraulic actuator. As a result, the machine member driven by the hydraulic actuator may shudder during a transition period while the pressures normalize. Such unexpected motion of the components driven by the hydraulic actuator are disturbing to the machine operator.
The existence of trapped pressure that may result in such undesired motion upon subsequent operation of the associated hydraulic actuator is referred to herein as a “trapped pressure condition.” The trapped pressure condition can be produced by the relative closing times of the inlet and outlet valves, a relief valve opening for one of the cylinder chambers but not the other chamber, thermal effects, and valve and cylinder leakage.
Prior manual spool values partially compensated for the effects produced by the trapped pressures by opening the return passage from workport to tank through the spool slightly before the passage from the supply line to another workport opened. However, this only compensated for the bootstrap effect.
Present day electrically controlled hydraulic functions use separate pairs of solenoid operated valves to connect each cylinder chamber to the fluid supply and return lines. This arrangement allows use of more metering modes that just standard powered extension and powered retraction of the cylinder provided by conventional spool valves. Specifically several regeneration modes are available by opening the solenoid operated valves in different combinations, as described in U.S. Pat. No. 6,775,974. Any one of several metering modes can be used to produce the same motion of the hydraulic actuator, with the particular mode to use depending upon the operating conditions at a given point in time. Providing the capability of selecting among a plurality of metering modes significantly complicates the alleviating the undesirable effects due to a trapped pressure condition.
Therefore, a mechanism still is needed to reduce or eliminate the shudder and other effects produced by pressure trapped in the hydraulic cylinder and ensure that the machine member will move only in the commanded direction.